In general, a spark plug used for igniting an internal combustion engine such an automotive engine includes a tubular metallic shell; a tubular insulator disposed in the bore of the metallic shell; a center electrode disposed in a forward end portion of the axial hole of the insulator; a metallic terminal disposed in a rear end portion of the axial hole; and a ground electrode whose one end is joined to the forward end of the metallic shell and whose other end faces the center electrode so as to form a spark discharge gap. Further, there has been known a spark plug including a resistor which is disposed in the axial hole between the center electrode and the metallic terminal so as to eliminate radio noise which would otherwise be generated when the engine is operated.
Incidentally, recent internal combustion engines for automobiles or the like have been required to produce a higher power and to operate with a higher efficiency, and development of a spark plug of a reduced size has been demanded in order to allow free design of engines and a reduction in the size of engines themselves. In order to reduce the size of a spark plug, the diameter of the bore of the insulator must be decreased. However, in the case of a spark plug designed in a conventional manner, decreasing the diameter of the insulator may deteriorate load life performance and decrease the fixing strength of the metallic terminal to the insulator.
Japanese Patent Application Laid-Open (kokai) No. 2009-245716 discloses a spark plug which can solve such a problem. In Japanese Patent Application Laid-Open (kokai) No. 2009-245716, there is recited a “spark plug characterized in that the diameter D of the electrically conductive glass seal layer is 3.3 mm or less, and the joint surface between the electrically conductive glass seal layer and the resistor is formed to have a curved shape.” Japanese Patent Application Laid-Open (kokai) No. 2009-245716 states that, the invention can provide a “spark plug which is enhanced in adhesion between the resistor and the electrically conductive glass seal layer, which is excellent in vibration resistance and load life performance of the resistor, and which has a reduced diameter” (see paragraph 0012).